This invention relates to optical switches and is particularly concerned with switches for switching optical signals composed of light of predetermined wavelengths, for example, Dense Wavelength Division Multiplexed (DWDM) optical signals used in optical telecommunications.
Optical transmission systems achieve their end-to-end connectivity by concatenating multiple spans between intermediate switching nodes to achieve an overall end-to-end path. When the end-to-end granularity of any given transmission path is a fraction of the capacity of a given optical carrier, time division multiplexing is used to share the overall bandwidth, mandating the use of electronic switching in the intermediate nodes. However, the availability of Dense Wavelength Division Multiplexing (DWDM), combined with the availability of high capacity ports on data switches and routers, has increased the demand for concatenation of individual spans to make end-to-end connections at the wavelength level.
DWDM optical networks transmit multiple channel signals on each optical fiber in the network; each channel signal is modulated light of a predetermined wavelength allocated only to that signal. The result is a plurality of optical carriers on each optical fiber, each optical carrier carrying a channel signal separated from other carriers in optical wavelength. Current DWDM optical networks typically convert channel signals into electrical signals at every switching node in the network because optical switches having sufficiently large enough port counts are not available. To convert the channel signals to electrical signals, transponders are used at every port of the switching node and for every channel wavelength. As DWDM signals become denser, that is, as the number of channels per optical fiber increases, the required accuracy of the transponders, and hence the cost, also increases. Moreover, as the number of ports per switching node increases, the required number of transponders also increases. Consequently, large networks carrying dense DWDM signals require many costly transponders and are therefore costly to build.
To overcome this problem it has been proposed to build large, purely optical switches in various forms, to reduce or eliminate the need for opto-electronic conversion in order to switch channel signals electrically. Some effort has gone into conceiving methods of building very large switches that offer full connectivity between all their ports. However, fabrication of these large optical switches has proven difficult.
Many attempts to create a large non-blocking optical switch use a large number of small switch modules to create a multiple stage switch. One example of this envisages building a 128 portxc3x97128 port switch out of three stages of multiple 16xc3x9716 crosspoint matrices, or a 512xc3x97512 port switch out of three stages of multiple 32xc3x9732 crosspoint matrices, in a three stage CLOS architecture. The above is based on the availability of 16xc3x9716 or 32xc3x9732 switch matrices in the form of Micro-Electro-Mechanical (MEM) switch matrices (e.g. xe2x80x9cFree-space Micromachined Optical-Switching Technologies and Architecturesxe2x80x9d, Lih Y. Lin, ATandT Labs-Research, OFC99 Session W14-1, Feb. 24, 1999). Other multi-stage approaches use smaller matrices and more stages. Even the 3 stage CLOS architecture is limited to 512-1024 switched wavelengths with 32xc3x9732 switch matrix modules, which, in today""s 160 wavelength per fiber DWDM environment, is only adequate to handle the output/input to 3-6 fiber pairs (480-960 wavelengths). Furthermore, the optical loss through each crosspoint stage (typically xcx9c5 dB with a 16xc3x9716 or 32xc3x9732 MEMs device) is compounded by the use of three stages, plus a complex interconnect, to provide switch losses in the range of 15-18 dB.
Such multi-stage switches, even at three stages, have significant problems. These problems include high overall optical loss through the switch, since the losses in each stage are additive across the switch, and there is the potential for additional loss in the complex internal interconnect between the stages of the switch. Size limitations in terms of the number of wavelengths switched can be overcome by going to a five stage CLOS switch, but this further increases the loss through the switch as well as adding to its complexity and cost. Using current loss figures, the loss through a 5-stage switch would be in the order of 25-30 dB. This amount of loss is at or beyond the operating link budget of modern high-bandwidth transponders. In addition, one of the major cost-centres is the cost of the MEMs switch modules (or other small matrix modules). Sensitivity of the overall switch cost to the cost of the MEMS modules is exacerbated by the fact that a CLOS switch requires a degree of dilation (i.e. extra switch paths) to be non-blocking and that each optical path has to transit three (or five) individual modules in series.
In U.S. Pat. No. 5,878,177 entitled xe2x80x9cLayered Switch Architectures for High-Capacity Optical Transport Networksxe2x80x9d and issued to Karasan et al., on Mar. 2, 1999, another approach is disclosed. This approach relies on providing signals received by a switching node with access to any route leaving the node, but not access to every signal path (fiber) on those routes. In this way, Karasan""s switching node avoids the large number of switch points that a fully interconnected, or fully non-blocking, switch fabric would require. Although this approach may be adequate at the node level, or even for small networks, it adds further complexity to network planning, which would become increasingly difficult with larger networks.
Some prior art approaches attempt to generate large, general purpose, non-blocking switches, which are then coupled to DWDM multiplexers for coupling into output fibers. This results in substantial waste of the capacity and capability of the non-blocking generic switches, since the DWDM multiplexers are themselves blocking elements on all their ports to any optical carrier except an optical carrier within the specific passband of that port of the multiplexer. Hence the non-blocking switch structure contains many crosspoints that direct specific input ports carrying a given wavelength to output ports that cannot support that wavelength, since it would be blocked in the WDM multiplexer. Such crosspoints cannot be used in operation of the switch, and this wasting of crosspoints makes inefficient use of expensive optical switching matrices.
Optical transmission networks that rely on electrical switching and electrical regeneration at intermediate nodes require one pair of transponders per wavelength channel at each intermediate switching node. Consequently, as the number of wavelength channels per fiber grows, the number of transponders and the resulting costs grow in proportion to the number of wavelength channels.
Optical transmission networks that rely on xe2x80x9copaquexe2x80x9d optical switching and electrical regeneration at intermediate nodes experience the same growth in transponder number and cost. (In xe2x80x9copaquexe2x80x9d optical switching, incoming optical signals are converted by transponders into different optical signals that are switched optically before being converted by further transponders to different optical signals for further transmission.)
However, in optically switched networks that use cascaded optical amplifiers to compensate for fiber loss on each span and for optical insertion loss of the optical switches, each optical amplifier simultaneously amplifies all wavelength channels on each fiber without the use of transponders. Consequently, the number and cost of the optical amplifiers does not grow with the number of wavelength channels per fiber, and the cost benefits of optically switched and amplified networks relative to electrically switched and regenerated networks increases with the number of wavelength channels per fiber.
Moreover, the cost advantages of optically switched and amplified networks over electrically switched and regenerated networks grow even faster as the maximum distance between electrical regeneration points grows, because optically switched and amplified networks can benefit from that increased optical reach by eliminating transponders. In contrast, electrically switched networks require a pair of transponders per wavelength channel at each intermediate switching point even if the optical range exceeds the distance between switching points.
Consequently, there is a substantial advantage in designing optical transmission networks such that the majority of wavelength channels can be routed end-to-end via optical switches and optical amplifiers, without the use of transponders on a per channel wavelength basis at intermediate sites or nodes. This leads to a need, previously unaddressed, for an optical cross-connect switch optimized for establishing per-wavelength paths from end-to-end, as opposed to a large opaque optical switching fabric designed to be located between banks of transponders.
This invention aims to provide an improved cross-connect switch which is well adapted for application to high capacity Wavelength Division Multiplexed (WDM) and Dense WDM (DWDM) transmission networks.
A first aspect of the invention provides a cross-connect switch comprising a plurality of switching matrices and a wavelength-converting inter-matrix switch. Each switching matrix has multiple input ports, multiple output ports, at least one inter-matrix input port and at least one inter-matrix output port. Each switching matrix is operable to switch an optical channel signal arriving on any input port to either any one of a plurality of the output ports or an inter-matrix output port. Each switching matrix is also operable to switch an optical channel signal arriving on any inter-matrix input port to an output port. Each switching matrix is further operable to switch optical channel signals having a respective distinct wavelength. The wavelength-converting inter-matrix switch is connected between the inter-matrix output ports of the switching matrices and the inter-matrix input ports of the switching matrices. The inter-matrix switch is operable to switch a channel signal arriving from any inter-matrix output port of any switching matrix to an inter-matrix input port of any of a plurality of other switching matrices. In switching a first channel signal having a first wavelength from an inter-matrix output port of a first switching matrix to an inter-matrix port of a second switching matrix, the wavelength-converting inter-matrix switch is operable to convert the first channel signal having the first wavelength to a second channel signal having a second wavelength.
Preferably, each switching matrix is operable to switch a channel signal arriving on any input port to any of the output ports. Furthermore, in such switches, the inter-matrix switch is operable to switch a channel signal arriving from any inter-matrix output port of any switching matrix to an inter-matrix input port of any of the other switching matrices. In this way, when networked together, such cross-connect switches provide increased flexibility in switching channel signals, thereby reducing the complexity of network planning as compared to other approaches.
This arrangement between the switching matrices and the intermatrix switch enables the assignment of each switching matrix to a respective channel wavelength of a WDM system. Channel signals having a particular wavelength can be routed through the cross-connect switch in the switching matrix assigned to that respective wavelength. Because this routing is through a single optical switching matrix, the optical loss can be relatively low.
When the next span of an end-to-end path does not have a particular channel wavelength available for a channel signal, the channel signal needs to be cross-connected to another channel wavelength. This cross-connection requires transponders to perform the necessary optical carrier wavelength conversion. This can be done by routing the channel signal, of a first channel wavelength, through a first switching matrix assigned to the first wavelength, to an inter-matrix output port of the first switching matrix. The channel signal is then routed from the inter-matrix output port of the first switching matrix to the wavelength-converting inter-matrix switch. The wavelength-converting inter-matrix switch converts the channel signal of the first wavelength to a channel signal of a second wavelength. The channel signal of the second wavelength is then routed to an inter-matrix input port of a second switching matrix, which is assigned to the second wavelength. The channel signal of the second wavelength is then routed to an output port of the second switching matrix, which completes the routing through the cross-connect switch to the next span, as required. Since wavelength conversion is only done as necessitated by network constraints, the cross-connect switch requires substantially fewer transponders than switches that convert all channel signals to electrical signals, or to a common channel wavelength, prior to switching.
Each switching matrix may have multiple inter-matrix output ports, and the wavelength-converting inter-matrix switch may comprise multiple switching elements connected in parallel. In this case, each inter-matrix output port of a particular switching matrix may be coupled to a respective one of the switching elements of the wavelength-converting inter-matrix switch. This arrangement provides multiple paths for routing a signal from one switching matrix through the inter-matrix switch to another switching matrix, thereby reducing potential for blocking in the inter-matrix switch.
Moreover, the physical interconnection between the multiple switching elements and the plurality of switching matrices may be accomplished efficiently by orienting the switching elements into a first set of parallel planes that are orthogonal to a second set of parallel planes into which the switching matrices have been oriented. For example, the switching matrices could be implemented on horizontally oriented switching cards and the switching elements fabricated on vertically oriented convertor cards, or vice versa. This physical arrangement allows the two orthogonal sets of parallel planes to be intersected by a third orthogonal plane, orthogonal to both sets of parallel planes, whereby each switching matrix of the second set of parallel planes can be brought into a proximal relationship, and optically interconnected, with each switching element of the first set of parallel planes. For example, a midplane representing the third orthogonal plane can be used to guide the switching cards and the convertor cards into a close physical arrangement, in which the switching and convertor cards can be optically interconnected with appropriate optical connectors on the cards and the midplane.
The inter-matrix switch may comprise at least one xe2x80x9caddxe2x80x9d input port and at least one xe2x80x9cdropxe2x80x9d output port. In this case, the inter-matrix switch is operable to couple an xe2x80x9caddxe2x80x9d input channel signal arriving at the xe2x80x9caddxe2x80x9d input port to an inter-matrix input port of any switching matrix, and to couple a channel signal arriving from an inter-matrix output port of any switching matrix to the xe2x80x9cdropxe2x80x9d output port. These features enable the cross-connect switch to xe2x80x9caddxe2x80x9d channel signals (i.e. to insert traffic signals at the cross-connect switch) and xe2x80x9cdropxe2x80x9d channel signals (i.e. extract traffic signals at the cross-connect switch) in addition to routing through channel signals.
The cross-connect switch may further comprise a plurality of wavelength division demultiplexers and a plurality of wavelength division multiplexers. Each demultiplexer is operable to separate an optical input signal into a plurality of output channel signals having respective distinct wavelengths. The demultiplexer applies each output channel signal to a respective input port of a respective switching matrix such that each switching matrix receives only channel signals having a respective distinct wavelength. Each multiplexer has a plurality of inputs, each respective input of each multiplexer being coupled to an output port of a respective switching matrix to receive a respective channel signal having a respective wavelength. Each multiplexer is operable to combine channel signals having distinct wavelengths into an optical output signal.
Such wavelength division demultiplexers and wavelength division multiplexers are normally associated with the cross-connect switch and may be packaged as part of the cross-connect switch. In this case, the wavelength division multiplexers and demultiplexers, implemented either separately or in combination on circuit cards, could have an orthogonal physical relationship with the plurality of switching matrices, in order to achieve efficiency in interconnection as described earlier. The demultiplexer receives an optical signal comprising multiple channel signals, each channel signal comprising an optical carrier at a respective distinct wavelength having a respective traffic signal modulated on the carrier signal. The demultiplexer separates the channel signals onto respective outputs for coupling to the switching matrices, each switching matrix receiving only channel signals at one of the distinct wavelengths. The multiplexer receives multiple channel signals, each having a different respective wavelength from respective switching matrices and combines the multiple channel signals for transmission on a single output fiber. In this arrangement, every cross-point of every switching matrix is usable, i.e. none of the cross-points route channel signals at a particular wavelength to a WDM multiplexer port that is unable to pass channel signals at that wavelength.
The wavelength-converting inter-matrix switch may comprise multiple optical receivers, multiple optical transmitters and an electrical switch connected between the optical receivers and the optical transmitters. The optical receivers are coupled to inter-matrix output ports of the switching matrices, and are operable to convert channel signals arriving from the inter-matrix output ports to electrical signals. The electrical switch is operable to switch electrical signals from any optical receiver to a plurality of the optical transmitters. The optical transmitters are operable to convert electrical signals to channel signals having predetermined wavelengths.
In most practical wavelength-converting inter-matrix switches, the electrical switch is operable to switch electrical signals from any optical receiver to any or substantially any optical transmitter. The electrical switch may be a single electrical switching element or multiple electrical switching elements connected in series or in parallel.
In this arrangement, the electrical switch is used to couple a receiver connected to a switching matrix assigned to a first wavelength to a transmitter operating at a second wavelength and connected to a switching matrix assigned to the second wavelength, thereby crossconnecting a channel operating at the first wavelength to a channel operating at the second wavelength.
Alternatively, the wavelength-converting inter-matrix switch may comprise an optical switch, and a plurality of optical transponders connected to the switch. Each optical transponder is operable to convert a channel signal having a first wavelength into a channel signal having a second wavelength. The optical switch is operable to couple a channel signal arriving from an inter-matrix output port of any switching matrix to an inter-matrix input port of any of a plurality of other switching matrices via an optical transponder.
The optical transponder may be a device having a receive half for recovering an information signal from the incoming wavelength channel, and a transmit half, having means to modulate the recovered information signal onto a light source of a specific, fixed or tunable, wavelength for output on a different wavelength channel. The optical switch may comprise a single optical switching element or multiple optical switching elements connected in series or in parallel for load sharing.
In most practical wavelength-converting inter-matrix switches, the optical switch is operable to couple a channel signal arriving from an intermatrix output port of any switching matrix to an inter-matrix input port of any or substantially any other switching matrix.
The optical switch may be coupled between the inter-matrix output ports and the optical transponders. In this arrangement, the optical switch is used to couple a first channel operating at a first wavelength to a selected transponder that converts the signal on the first channel to a signal at a second wavelength. The transponder is connected to an intermatrix input port of the switching matrix that is assigned to the second wavelength.
Alternatively, the optical switch may comprise plural optical switching stages and the optical transponders may be coupled between optical switching stages. For example, the optical switch may comprise a multistage optical CLOS switch. The relatively high insertion loss of a multistage optical switch is acceptable in the inter-matrix switch because the inter-matrix switch includes transponders that restore the optical signal level as they convert an optical signal at one wavelength to an optical signal at another wavelength. However attention must be paid to an overall system loss budget to keep all components operating within their specified range.
Some or all of the optical transponders may be tunable to transmit channel signals of selectable distinct wavelengths. The use of tunable transponders reduces the number of transponders that need to be provided to allow for all possible wavelength conversion possibilities. Each tunable transponder can be provisioned remotely for any of a number of wavelength channels without requiring a visit to the switching site to physically provision a wavelength channel. It can be demonstrated statistically that a number of tunable transponders can provide more combinations of channel configurations than the same number of fixed wavelength transponders. Moreover, the use of tunable transponders reduces the number of different transponder types that must be stocked and inventoried.
However, tunable transponders are more expensive than fixed wavelength transponders and currently have limited tuning range. Consequently, some or all of the transponders may be fixed wavelength transponders that are operable to transmit channel signals of a single wavelength. Alternatively the tunable transponders may be arranged in groups, each group covering the ports associated with a specific wavelength band.
Another aspect of the invention provides an optical switching matrix comprising first and second pairs of switching elements and a plurality of optical combiners. Each pair of switching elements comprises a first switching element and a second switching element. Each switching element comprises a rectangular substrate having a plurality of input ports on a first side, a first plurality of output ports on a second side opposite the first side and a second plurality of output ports on a third side adjacent the first side and the second side. Each switching element further comprises a plurality of optical diverters aligned between each input port and a corresponding output port on the second side. Each diverter is aligned with a respective output port on the third side and is movable from a first position, in which the diverter allows an optical signal incident from the input port to propagate in a direction toward the respective output port on the second side, to a second position, in which the diverter diverts an optical signal incident from the input port toward a respective output port on the third side. For each of the first and second pairs of switching elements, each input port of the second optical switching element is optically coupled to a respective output port of the first optical switching matrix. Each combiner is coupled to a respective output port of the first pair of optical switching elements and to a respective output port of the second pair of optical switching elements.
Construction of larger switching matrices by assembly of smaller switching matrices as described above, may be attractive until switching matrices of the desired port count are readily available at attractive prices. Moreover, the ability to assemble larger switching matrices from smaller switching matrices enables modular construction of cross-connect switches so that the size of the switch (and its installed cost) can grow gracefully with capacity demands.
Accordingly, another aspect of the present invention provides a plurality of switching matrices, each switching matrix being assignable to a respective channel wavelength, as well as having multiple input and output ports and at least one pair of inter-matrix input and output ports. Additionally, each switching matrix has an expansion port for coupling to an input port of an extension-switching matrix, which is also assignable to the respective channel wavelength. In this way, each switching matrix can be extended, thereby increasing its switching capacity and further increasing the switching capacity of the cross-connect switch that includes the extended switching matrices. For example, the size of a switching matrix could originally be 32xc3x9732 and an extension switching matrix of the same size could be coupled to it, via the expansion port, to result in an extended switching matrix of size 32xc3x9764. A cross-connect switch having a plurality of these extended switching matrices could be coupled, via optical combiners, to another cross-connect switch having a similar plurality of extended switching matrices. This would result in a combined cross-connect switch with double the switching capacity and port count of either of the original cross-connect switches.
Another aspect of the invention provides a wavelength-converting switch for interconnecting optical switching matrices of an optical cross-connect switch, the wavelength-converting switch comprising an optical switch and a plurality of optical transponders connected to the switch. Each optical transponder is operable to convert a channel signal having a first wavelength into a channel signal having a second wavelength. The optical switch is operable to couple a channel signal arriving from an inter-matrix output port of any switching matrix to an inter-matrix input port of any of a plurality of other switching matrices via an optical transponder.
The wavelength-converting switch can be used in the construction of some embodiments of the cross-connect switch described above.
Another aspect of the invention provides a switching fabric for an optical cross-connect switch. The switching fabric comprises a plurality of optical switching matrices. Each switching matrix has multiple internode input ports and at least one intra-node input port for receiving incoming optical channel signals, the incoming optical channel signals having a wavelength that is particular to that particular switching matrix. Each switching matrix also has multiple through output ports and at least one intra-node output port. Each switching matrix is operable to switch optical channel signals arriving on any input port to any of a plurality of the through output ports and the intra-node output port.
In most practical switching fabrics, each switching matrix will be operable to switch optical channel signals arriving on any input port to any or substantially any of the output ports.
The switching fabric may further comprise an add/drop multiplexer coupled to the intra-node input port and intra-node output port of each switching matrix. The add/drop multiplexer is operable to couple, to the intra-node input port of any switching matrix of the plurality of switching matrices, optical channel signals having the wavelength that is particular to that switching matrix. The add/drop multiplexer is also operable to receive, from the intra-node output port of any switching matrix of the plurality of switching matrices, optical channel signals having the wavelength that is particular to that switching matrix.
Another aspect of the invention provides a method of cross-connecting optical channel signals at an optical cross-connect switch comprising a plurality of switching matrices. The method comprises coupling each optical channel signal having a particular wavelength to an input port of a particular switching matrix assigned to that particular wavelength, and switching the optical channel signal in the particular switching matrix to an output port selected according to a desired cross-connection of the optical channel signal.
The optical channel signal may be switched to an intra-node output port of the particular switching matrix when the optical channel signal is to be cross-connected to an optical channel having a wavelength other than the particular wavelength of the optical signal. In this case, the optical signal may be coupled from the intra-node output port to a wavelength converter for conversion to an optical channel signal having another wavelength. The optical signal at the other wavelength can be coupled to an intra-node input port of another switching matrix, the other switching matrix being assigned to that other wavelength. The other switching matrix can switch the optical channel signal to an output port selected according to the desired cross-connection of the optical channel signal.
The optical channel signal may also be switched to an intra-node output port of the particular switching matrix when the optical channel signal is to be dropped at the cross-connect switch.
According to another aspect of the invention, the invention provides an optical connection system for optically connecting circuit cards via a midplane. The interconnect includes a first connector for connecting a first plurality of optical fibers coupled to a first circuit card. The first connector has a first mounting means for mounting the first connector adjacent an edge of the first circuit card.
A second connector is included for connecting a second plurality of optical fibers coupled to a second circuit card. The second connector has a second mounting means for mounting the second connector adjacent an edge of the second circuit card. A first mating insert, disposed in the first connector, is included for aligning the first plurality of optical fibers in an optically coupled relationship with the second plurality of optical fibers. A second mating insert, disposed in the second connector, is included for aligning the second plurality of optical fibers in an optically coupled relationship with the first plurality of optical fibers. Finally, an alignment ferrule is included for mounting in an opening in the midplane. The alignment ferrule has an aperture for receiving the first mating insert on one side of the alignment ferrule and the second mating insert on the other side of the alignment ferrule. The aperture in the alignment ferrule is oriented to pass through the opening in the midplane when the alignment ferrule is mounted therein.
The alignment ferrule provides a means to align the mating inserts such that the final alignment features of the connectors, in this case a pair of guide pins with corresponding sockets, can engage and provide the final alignment of the optical fiber ends at the faces of the mating inserts. The final alignment features provide translational alignment along orthogonal axis parallel to the faces, as well as rotational alignment about an axis perpendicular to the faces such that the multi-fiber ribbon cables can be optically aligned.
According to yet another aspect of the present invention there is provided an optical network comprising at least one optical cross-connect switch wherein optical fibers couple the optical switching matrices to the optical network via the input and output ports. Alternatively, or additionally, where the optical cross-connect switch includes the wavelength division multiplexers and demultiplexers, optical fibers couple the wavelength division multiplexers and demultiplexers to the optical network for respectively transmitting and receiving optical output and input signals.
According to still another aspect of the present invention there is provided a method of upgrading an optical cross-connect switch having a plurality of switching matrices, each switching matrix assigned to a respective channel wavelength and having multiple input and output ports, the method comprising the steps of: providing each switching matrix with an expansion port; providing a plurality of extension switching matrices, each extension switching matrix having multiple input and output ports; and coupling a respective extension switching matrix to each switching matrix, via the expansion port and at least one of the input ports of the respective extension switching matrix, to form a plurality of expanded switching matrices.
Additionally, the optical cross-connect switch may be upgraded further by providing another similarly upgraded optical cross-connect switch having a plurality of the expanded switching matrices; and coupling each output port of an expanded switching matrix of the optical cross-connect switch to a respective output port of an expanded switching matrix of the another optical cross-connect switch.
Other aspects of the invention comprise combinations and subcombinations of the features described above other than the combinations described above.